1. Field of the Invention
The present invention relates to a liquid crystal display device, a driving method thereof, and an electronic device. More particularly, the present invention relates to a liquid crystal display device suitably used for the purpose of displaying moving pictures, a driving method thereof, and an electronic device incorporating such a liquid crystal display device.
2. Description of the Related Art
In recent years, liquid crystal display devices (hereinafter referred to as “LCDs”) have been in wide use. The mainstream has heretofore been TN-type LCDs in which nematic liquid crystal having a positive dielectric anisotropy is employed in a twist alignment. However, TN-type LCDs have a problem in that they have a large viewing angle dependence associated with the orientation of liquid crystal molecules.
Therefore, so-called alignment-divided vertical alignment type LCDs have been developed to improve on the viewing angle dependence, and applications thereof are becoming more and more widespread. For example, Japanese Patent No. 2947350 discloses an MVA-type liquid crystal display device, which is one species of alignment-divided vertical alignment type liquid crystal display device. The MVA-type liquid crystal display device is an LCD which performs display in a normally black (NB) mode by using a vertical alignment type liquid crystal layer which is provided between a pair of electrodes. Domain restriction means (e.g., slits or protrusions) are provided so that liquid crystal molecules in each pixel will lean or incline in a plurality of different directions under an applied voltage.
Recently, there has been a rapidly increasing need to display moving picture information, not only on liquid crystal television sets, but also on PC monitors and portable terminal devices (such as mobile phones or PDAs). In order to display high-quality moving pictures on an LCD, it is necessary to reduce the response time (i.e., increase the response speed) of the liquid crystal layer, and it is a requirement that a predetermined gray scale level be reached within one vertical scanning period (typically one frame).
As for MVA-type LCDs, Japanese Patent No. 2947350 discloses, for example, that the response time between black and white can be reduced to 10 msec or less. It is also described that, by providing regions differing in distance between protrusions within each pixel to give regions with different response speeds, improvement in apparent response speed can be attained without reducing the aperture ratio (see FIGS. 107 to 110 of Japanese Patent No. 2947350, for example).
On the other hand, as a driving method for improving the response characteristics of an LCD, there is known a method (referred to as “overshoot driving”) that involves applying a voltage (referred to as an “overshoot voltage”) which is higher than a voltage (a predetermined gray scale voltage) corresponding to a gray scale level that needs to be displayed. By applying an overshoot voltage (hereinafter referred to as an “OS voltage”), the response characteristics in gray scale display can be improved. For example, Japanese Laid-Open Patent Publication No. 2000-231091 discloses an MVA-type LCD which operates by overshoot driving (hereinafter “OS driving”).
However, through detailed study, the inventors of the present invention have found a new problem which occurs when OS driving is applied to an alignment-divided vertical alignment type LCD, such as the aforementioned MVA-type LCD. This problem will be described with reference to FIG. 11.
FIG. 11 is a graph illustrating changes in transmittance over time when OS driving is performed for an MVA-type LCD which performs display in a normally black mode. In FIG. 11, the solid line represents transmittance corresponding to a target gray scale level, whereas the dotted line and the dot-dash line show transition of the actual transmittance.
In general, there are two types of response of a liquid crystal layer: “rise” and “decay”. A “rise” is a change in the display state in response to an increase in the voltage applied across the liquid crystal layer. A “decay” is a change in the display state in response to a decrease in the voltage applied across the liquid crystal layer. In an LCD of a normally black mode, “rise” corresponds to an increase in transmittance, whereas “decay” corresponds to a decrease in transmittance.
FIG. 11 illustrates a case where response occurs in the order of a decay and then a rise. As shown by the dot-dash line in FIG. 11, it is preferable that a transmittance corresponding to a target gray scale level be reached within one vertical scanning period. However, in an actual LCD, as shown by the dotted line, transmittance may not decrease to the transmittance corresponding to the target gray scale level within one vertical scanning period, during a decay response. When an OS voltage for a rise response is applied in this state, the transmittance will become higher than the transmittance corresponding to the target gray scale level, thus causing a substantial shift to the white side (hereinafter referred to as “white shift”).